Stimulated Raman scattering can be viewed as a modulation process produced by molecular vibrations. In the process, the application of an incident radiation, i.e., pump radiation, results in the generation of upper and lower sidebands. In particular, it is the frequency shift to longer wavelengths that is of special interest in that it provides a means for generating coherent radiation in the 1 .mu.m to 1.8 .mu.m range. This range includes the 1.3 .mu.m and 1.55 .mu.m wavelengths which are of particular interest in long distance optical transmission systems.
Raman signal sources currently available include silica Raman fiber lasers which are pumped by mode locked and Q-switched Nd:YAG lasers having 100 picoseconds pulse widths. While silica fiber lasers are reliable and simple, their outputs are not continuously tunable with currently available pump sources. In addition, the silica Raman shift is relatively small (i.e., 400 cm.sup.-1). (See, for example, U.S. Pat. No. 3,705,992, issued Dec. 12, 1972.) As a consequence, one cannot obtain very short (i.e., 1 picosecond) Raman shifted output pulses in the infrared region inasmuch as the short picosecond sources are only available in the visible region.
Alternative sources are the tunable F-center lasers, which are more difficult to operate and are fairly short lived, and the Raman gas lasers. The gain medium in the latter are one or more gases under pressure. Typically, each gas has a different Raman shift, thereby providing a plurality of output signals over the band of interest. The problem with these arrangements resides in the small interaction path which, in turn, results in only modest output power levels unless very large pump powers are used.